Many releasable door hold-open devices are known in the art for automatically releasing a held-open door when a predetermined condition is detected, such as the existence of smoke or heat. These devices are often used with fire doors, which must be closed in the event of a fire to inhibit spreading of the fire.
Most prior devices use a solenoid that must be continually energized to hold open the door. Examples of such devices are disclosed in representative U.S. Pat. Nos. 3,729,771, 3,771,823, 3,905,063 and 4,040,143. When smoke or heat is detected, or if current to the device is interrupted, the solenoid is de-energized, thus automatically releasing the door and allowing it to close under the force of a door closer. This implementation has the distinct disadvantage of requiring the electric actuator to be continuously energized to hold the door in the open position. A continuously energized actuator draws a large continuous electrical current, which wastes electricity, decreases the life of the actuator and makes battery operation impractical.
The method of wiring such prior devices presents additional problems. A continuous current requires wiring the door hold-open device to the main power supply of the building, which is expensive, particularly for older building which must be retrofitted. Since the electric actuators in many prior devices do not run off a typical 120 volt electrical supply, special transformers and wiring are required, again at additional cost. In addition, when multiple door hold open devices are wired to a common fire or smoke alarm, the wire connections for different devices are often different. For example, the first, last and intermediate devices in a series of devices must often be wired differently. This causes confusion and frequently results in incorrect wire connections.
Since prior hold-open devices are usually connected to a central electrical source, the devices are subject to damage from power surges caused by lightning strikes and power surges from the electric supply.
Another problem faced by prior hold open devices occurs when they are wired to a common smoke or fire detector. An alarm condition may result in the release of all doors wired to the system, even though many of those doors may not be near the fire or smoke.
Because prior hold-open devices typically employ a remote power source, doors will be released if a power failure occurs, even if no fire is present. After the power is restored, each unit having a smoke detector must be individually reset before it will hold the door open. This is an inconvenient and time-consuming task.
Finally, many prior hold open devices can accommodate only one type of door-closer unit. This increases the cost of the unit because the purchaser must also pay for the closer unit. There is a felt need for a universal hold-open device that can be installed in conjunction with any type of preexisting door closer.
U.S. Pat. Nos. 4,506,407, 4,656,690 and 4,715,146, disclose automatically releasable hold-open devices that allow a door to latch open while the actuating device is not energized. However, these devices suffer from an even greater defect. Should the power supply to the device fail, a latched-open door will not be released. Most building codes require that door hold-open devices include a fail-safe feature to automatically release a door upon interruption of the power source. While a battery power supply is practical for this type of hold-open device, it is not safe because the battery may, without warning, become so discharged that it cannot provide sufficient electromotive force to release the door. Thus, if the power fails before the device detects smoke or heat, the door will remain open allowing a fire to spread more rapidly.